1. Field of the Invention
The present invention is directed to underground heat exchange systems; to apparatus and methods for drilling wellbores for such systems using sonic drilling systems; and to installing underground heat exchange systems.
2. Description of Related Art
The prior art discloses a wide variety of systems for circulating water or refrigerant through pipes in the earth with)the earth serving as a heat sink or heat source heat transfer between the fluid in the pipes and the earth. The prior art discloses numerous in-ground heat exchanger systems; and grouting systems (see, e.g. U.S. Pat. Nos. 5,435,387; 5,244,037; 5,261,251; 5,590,715; 5,758,724; 6,250,371; 6,041,862; and 6,276,438; and pending U.S. application Ser. No. 10/387,125 filed Mar. 12, 2003; and the references cited in these patents and application)—all of which patents, applications therefor, references and application are incorporated here fully for all purposes.
Often the energy efficiency of a geothermal heat pump system is far superior to traditional HVAC alternatives of air source heat pumps, boilers, chillers, furnaces, cooling towers, etc., in many cases requiring fifty percent less energy to operate. One reason why this energy efficient GHP heating/cooling technology is not more widely used is that the initial installed cost is usually higher than for traditional HVAC systems. Often fifty percent or more of the initial cost of a geothermal heat pump system is for constructing an earth loop heat exchanger. Thus it is desirable that the cost of this heat exchanger be kept to a minimum. However, when geological conditions are unusually difficult, drilling and completion costs can dramatically increase, and the cost of installing a geothermal heat pump system can become prohibitive. Thus there is a need, recognized by the present inventor, for a method for the economical construction of a earth heat exchanger which would make possible the widespread use of this energy efficient and environmentally friendly geothermal heat pump technology.
With many prior art systems, the construction of an earth loop heat exchanger has included drilling a borehole 150 to 500 feet deep, inserting a small diameter (usually 0.75 inch to 1.25 inch diameter) polyethylene pipe loop, and grouting the loop in place. Trenches are then dug between such multiple boreholes and earth loop ends are manifolded together in parallel, connected to a common header, and then connected to heat pumps. Water is then circulated through this closed loop system, and heat is absorbed from or transferred to the earth, as the heat pumps require. If so called “direct exchange” heat pump technology is utilized, often copper refrigerant lines are placed in direct contact with the earth, and the heat exchange is accomplished directly with the earth instead of through a secondary water-to-refrigerant heat exchange. Both the more widely used “water source” heat pump technology systems and the direct exchange heat pump technology are available. Both systems are similar insofar as the drilling and completion problems with the earth heat exchangers are concerned.
One typical earth loop heat exchanger design for a small school might require the drilling of 100 holes to a depth of 300 feet, while a large school might require 1000 such holes. High drilling rates and consistent production is preferred for keeping down the costs of the earth heat exchanger. One type of drilling arrangement used to build a geothermal heat exchanger system provides a large number of holes that are relatively deep. Rapid mobilization and set-up between closely spaced holes, fast drilling rates of penetration, and hole stability are critical. However, the drilling technology used in other drilling disciplines is not well-suited or optimized for such geothermal applications. For example, quarry drilling, seismic drilling, and construction drilling all require large numbers of holes in a relatively compact geographic area, but the holes required are relatively shallow in depth, often less than 100 feet deep. In water well drilling and geotechnical investigation drilling, the holes are drilled deeper, but they are relatively small in number. With ancillary operations such as sampling, logging, setting pumps, installing controls and monitoring devices, etc., the speed at which water wells or environmental wells are drilled becomes less critical than it is on geothermal projects.
In addition, many drill rigs are usually designed to drill optimally in only one specific type of formation. Unconsolidated formations such as sand/clay/gravel formations are usually drilled best using mud rotary techniques. The mud rotary drill system utilizes a mud mixing/circulation system incorporating duplex or triplex mud pumps. The drilling mud circulation system transports the drilled spoils out of the hole and also serves to help keep the drilled hole from collapsing. If hard or rock conditions are encountered, drilling progress slows down significantly. If extremely unstable soil conditions are encountered, the mud weight must be regulated carefully to prevent hole collapse and/or to prevent “mining” or over-excavation from occurring.
Constant mud regulation can slow drilling progress significantly. The production rate in mud rotary applications can vary widely and is affected by the formation geology and by the expertise of the driller. The driller is required to know when to thicken up, thin out, or weight up drilling mud; which drill bit to select; what rotation speed to use; how much circulation time is adequate; what additives to put in the mud; and how much pull-down force to apply, etc. These variables can change significantly as the lithology through which drilling is being done changes with increasing depth. In addition, mud drilling can significantly increase the thermal resistance of the borehole in the near-wellbore area by building a wall-cake of poorly thermally conductive bentonite drilling mud—thus requiring additional holes to be drilled.
At another extreme, hard formations such as limestone, sandstone, granite, etc. are usually drilled most economically with downhole hammers powered by compressed air. Even where drilling conditions are predominantly rock, there is usually a layer of unconsolidated overburden on top of the rock which must be penetrated and stabilized before the downhole hammers can effectively begin to drill the rock. If the overburden is very soft or unstable, the top portion of the hole must be supported with casing pipe, usually steel, at least for a depth down to the rock. If the overburden is not stabilized, the top portion of the hole will erode or be “mined out” as the rock is drilled, in extreme cases causing the drill rig to capsize. The expense of traditional overburden drilling and stabilization has made many geothermal projects' cost prohibitive.
Often it is only necessary to case a geothermal borehole temporarily. Once the heat exchanger pipe has been inserted and grouted in place, the casing can be removed. Not only is permanent casing expensive, it can also inhibit the thermal transfer of heat between the heat exchanger pipe and the earth. Even where carousels or magazines automatically handle drill pipe, any casing is often handled manually—using slings, ropes, and cables. In many casing operations, the casing pipe sections are welded together while tripping into the hole and cut apart with a torch as they are extracted from the hole. Besides being a physically demanding job requiring additional labor, the process of loading heavy and clumsy casing pipe is dangerous to personnel. The logistics of handling the casing pipe at the surface usually dictate the use of additional surface equipment such as forklifts, cranes, boom trucks, etc., thus incurring additional equipment rental and labor costs.
It is widely known to drillers skilled in the art that stabilizing a borehole by using steel or other casing is a good and reliable way to ensure borehole integrity. But it is also just as commonly acknowledged that steel casing is usually the most expensive method of stabilizing a borehole, and, because of its high cost, casing is used only as method of last resort. One standard practice of installing casing is to drill a borehole by conventional means, and then insert the casing in the pre-drilled hole. This method presumes that the hole will stand up long enough to insert the casing to the desired depth. However, if the geology is very unstable, which is the primary reason casing is being inserted in the first place, the pre-drilled hole can collapse before casing insertion can be completed. To compensate for this condition, many drilling procedures and special tools have been developed. One of the well-known concepts is simultaneous drilling while casing, in which the casing is advanced as the hole is being drilled. In some prior art systems two sets of tubulars of different diameters, i.e. drill pipe and casing pipe, are simultaneously advanced one inside the other. Often, two independent rotary drillhead mechanisms are employed, each rotating the casing and drill pipe in opposite directions, and each rotary head capable of independent longitudinal travel along a common drill mast. It can be a complex task to effectively insert drill pipe inside casing, present them to two separate drill heads, clamp the different size diameters of pipe while making up and/or breaking out the threaded connections, rotate the pipe and casing in opposite directions while moving them up and down independently. The process is inherently expensive because the machine is very complicated and additional labor and equipment is required to handle the pipe and casing. Drilling production, while reliable, predictable, and fast when compared to first drilling a hole and then setting the casing afterward, is still relatively slow compared to drilling without casing.
Another prior art method of setting casing utilizes a casing hammer to hammer the casing into the ground. Once the casing is hammered in place, the earth inside the casing is drilled out. However, due to the rapid build-up of skin friction between the earth and the casing, the physical limitations of the casing, and the high power required by the hammer, casing driving is slow, and often can only be accomplished to fairly shallow depths. The casing also frequently becomes stuck and/or damaged.
Although sonic drilling principles have been well known for many years, application of these principles has been largely dedicated to specialized areas of geotechnical investigation, mineral sampling, environmental sampling, and monitoring well construction. In these types of drilling, relatively undisturbed core samples of earth are captured inside the casing and recovered for analysis as the drilling progresses. Since the primary emphasis in these types of drilling is on gathering quality soil samples, speed is not essential, and the potential for using sonic drilling for relatively high-speed geothermal borehole production has not been previously considered. In addition, the sonic drillhead and tooling used in sonic drilling can be substantially more expensive than traditional mud rotary or air drilling equipment; and, since geothermal drilling has been normally considered to be very “cheap” drilling work, the investment in “high technology” sonic equipment to drill boreholes has often been discounted as economically prohibitive.